1. Field of the Invention
This invention relates to a data processor and particularly to a data processor adapted for use in a printer or a display device provided with a masking function whereby the selected data for recording or display is masked.
2. Description of the Prior Art
The conventional data processing machine is usually provided with a data output device for performing the function of recording or displaying processed data, and, in the recording or display of such data in a determined format, it is frequently desirable to conceal or mask a selected part of such data.
In the case of a printer as an example of a data output device, an impact line printer is capable of easily preparing multiple copies, and in such an instance it is already well known and widely utilized to obtain masked multiple copies wherein selected data are masked by suitably positioning the copying carbon.
On the other hand a non-impact printer is not usually capable of producing multiple copies, and it is therefore almost impossible to perform masking, particularly on the recording paper to be used on the printer.
Such a non-impact printer can be exemplified by a laser beam printer which will be further explained in the following, with particular reference to FIG. 1 schematically illustrating the fundamental construction of such a laser beam printer. The following description will be limited to the outline since the control of such a laser beam printer is explained in detail in the U.S. patent application Ser. No. 616,675 now U.S. Pat. No. 4,059,833, issued Nov. 22, 1977 and assigned to the assignee of the subject invention.
Referring to FIG. 1, a laser beam generated by an laser oscillator 1 is introduced through a mirror 2 to an input aperture of a modulator 3. Mirror 2 is utilized for deflecting the light path to minimize the necessary space and can be dispensed with if space is not an important factor.
Modulator 3 can be an acousto-optical modulating element utilizing the known acousto-optical effect or an electro-optical element utilizing the known electro-optical effect. The laser beam is subjected to intensity modulation in modulator 3 according to the input signal thereto.
When the laser oscillator is a semiconductor laser, a gas laser of the type capable of current modulation or an internal modulation laser provided with a modulating element in the light path, the modulator 3 may be dispensed with so that the laser beam is directly guided to a beam expander 4.
In the beam expander, the laser beam is expanded in diameter while maintaining the state of parallel flux. Then, the expanded laser beam is guided to a polygonal rotary mirror 5 provided with one or plural mirror faces. Rotary mirror 5 is supported by a high precision bearing (for example an air bearing) and driven by a motor 6 (for example, a hysteresis synchronous motor or a DC servo motor) at a constant speed, and the laser beam is deflected in a horizontal sweeping motion by means of the rotary mirror 5. The deflected beam is focused, by means of an imaging lens 7 having f-.theta. characteristic, as a spot on a photosensitive drum 8. In an ordinary imaging lens, the image position r on the focal plane is related with the incident angle .theta. of the beam through an equation: EQU r=f.multidot.tan .theta. (1)
where f is the focal length of the imaging lens. In the foregoing apparatus the incident angle, to the imaging lens 7, of the laser beam 12 reflected by the polygonal rotary mirror 5 changes as a first-order function of time, and the displacing speed of the spot on the photosensitive drum 8 is not constant but changes non-linearly. More specifically the displacing speed increases as the incident angle becomes larger. Consequently if a series of spots is recorded on the photosensitive drum 8 by turning on the laser beam at constant time intervals, the separation between the spots will become larger at both ends than in the center.
In order to avoid such a phenomenon, the imaging lens 7 is designed to have characteristics: EQU r=f.multidot..theta. (2)
and such a lens is called an f-.theta. lens.
Also in the case of focusing a parallel light beam into a spot by means of an imaging lens, the minimum diameter d.sub.min of the spot is given by the following equation: EQU d.sub.min =.epsilon.f.lambda./A (3)
wherein:
f: focal length of imaging lens
.lambda.: wavelength of light used
A: incident aperture of imaging lens, or diameter of incident beam if smaller
.epsilon.: constant determined by the beam shape
and a smaller spot diameter can be obtained by increasing A for given f and .lambda.. The above-mentioned beam expander 4 is utilized to obtain this effect, and can be dispensed with if a desirable spot diameter d.sub.min can be obtained with the original beam diameter of laser oscillator.
The beam detector 18 consists of a small incident slit and a photoelectric converting element with a rapid response time (for example a PIN diode), and detects the position of the laser beam 12 in sweeping motion. The detection signal of detector 8 is utilized for determining the timing for the start of input signals into the modulator 3 for providing the photosensitive drum with the desired light information. In such a manner it is rendered possible to significantly reduce the aberration of the signal resulting from the error in the division of the reflecting faces of rotary mirror 5 and from the uneven rotation thereof thereby assuring an image of good quality and increasing the tolerance required for the rotary mirror 5 and the drive motor 6 thereby allowing a lower production cost.
As explained in the foregoing, the modulated and deflected laser beam 12 is directed to the photosensitive drum 8, and the resulting image is rendered visible by an electrographic process, then transferred to plain paper where it is fixed to obtain a hard copy.
In the following, an explanation will be given of the printing unit 20 with reference to FIG. 2.
An an example of electrographic process applicable to such a printing unit is described in the U.S. Pat. No. 3,666,363 assigned to the assignee of the present application, wherein, a photosensitive plate 8 essentially composed of an electroconductive substrate, a photoconductive layer and an insulating layer is first subjected to electrostatic charging with a corona discharger 9 to form a uniform positive or negative charge on the surface of the insulating layer thereby capturing an electrostatic charge of an inverted polarity at the interface between the photoconductive layer and the insulating layer or within the photoconductive layer. Then the charged insulating layer is subjected to irradiation of laser beam 12 simultaneously with an alternating corona discharge by an alternating corona discharger 10 thereby forming a potential difference pattern on the surface of the insulating layer according to the intensity pattern of the laser beam 12. The insulating layer is then subjected to a flush exposure to form an electrostatic image of elevated contrast on the surface of the insulating layer, which image is rendered visible in a developing device 13 by means of a developer essentially consisting of charged colored particles. The thus obtained visible image is transferred to a transfer material 11 such as paper by means of an internal or external electrical field and fixed by a fixing means 15 utilizing an infrared lamp or a hot plate to complete an electrographic print. Meanwhile the surface of the insulating layer is cleaned by a cleaning device 16 to permit repeated use of the photosensitive plate 8.
In a printer of the above-mentioned composition, the data corresponding to one page are stored in a memory device called a page buffer, and multiple copies of identical content can be obtained by repeatedly printing the data stored in the page buffer. For rapid printing, there is in fact no technical difficulty in producing multiple copies. However a difficulty is encountered in the case of obtaining masked multiple copies, wherein selected data are masked, unlike the impact printer.
The above-mentioned laser beam printer is strongly characterized as a page printer, and, in the case of producing multiple copies, it is therefore necessary to collect the data independently for each copy if there is even a slight difference between the contents of the first copy and of the second copy. This fact therefore required an increase in the capacity of buffer memory or storage and to a lowered output due to the time loss required for data transfer. For the case of an off-line printer, the data recording has to be conducted by memorizing the data for the first copy and those for the second copy separately into the page buffer from a data file such as a magnetic tape, and such process is quite wasteful and inefficient despite the fact that the masking is required only in a very small portion of the data to be recorded. Thus complication of the apparatus and an increase in the cost are unavoidable if it is necessary to increase or decrease the masked portion.
Such a drawback is not limited only to a laser beam printer but also plagues any non-impact printer or data output device.